Theme editor: Unable to edit files more than 1-level deep

Description

In [41806] for #6531 the logic for gathering the list of $allowed_files was modified to allow for a unlimited depth for listing out files to edit. However, the same change was not then likewise applied to wp_edit_theme_plugin_file() when actually validating the file path to edit. The result is that a user can view the code any file in a theme but they cannot save any edits they make. The logic in wp_edit_theme_plugin_file() needs to be updated to match the code in theme-editor.php.

Theme Editor: Ensure files listed recursively can be both viewed and edited.

Prevent edits to 2-level deep theme files from returning a disallowed_theme_file error when attempting to save an edit. Aligns logic for gathering $allowed_files in theme-editor.php for listing files with the validation logic in wp_edit_theme_plugin_file().